什么时候计算 lambda 函数中的表达式？

Question

Given this little snippet: a and b are not known at compile time. Map is a mapping function for the vector, mapping the lambda function over each element of bar. Supposing fooify() is called ... Will a/b be evaluated as a constant before executing map() or will it be evaluated every time the lambda is called?

Foo::fooify(vector<float>& bar){
  float a = getA();
  float b = getB();
  map(bar, [&](float c){c*(a/b);});
}

Answer 1

I suspect a/b will be evaluated in every call to the lambda function. You can make sure that it is evaluated only once by using:

Foo::fooify(vector<float>& bar){
  float a = getA();
  float b = getB();
  float ab = a/b;
  map(bar, [](float c){c*ab;});
}